The present invention relates to a solar cell of silicon semiconductor. More particularly, the invention relates to a silicon solar cell having a novel and unique structure which is capable of exhibiting high efficiency for the conversion of the solar energy into electric energy despite the outstandingly low costs for the preparation thereof.
It should be mentioned that, while the energy traditionally consumed by mankind is obtained almost exclusively from fossil fuels such as coal, petroleum and natural gas, these carbon-based energy sources are at the risk of exhaustion in the near future and, in addition, responsible for the problems of environmental pollution such as the problem of global warming as recognized as a serious issue in recent years. Under the above mentioned background situation, it is eagerly desired to develop a means for the direct conversion of solar energy to electric energy so that solar cells utilizing a silicon semiconductor are highlighted as a promising means for the solar power generation. Despite the large accumulation of results obtained by the research and development efforts undertaken heretofore in countries world-wide, the largest factor which prohibits the practical utilization of silicon solar cells is the high cost of the electric power generated by using them as compared with conventional means for commercial power generation so that the efforts for the development of solar cells are directed to the cost reduction in the manufacture of silicon solar cells.
A conventional silicon solar cell typically has a structure in which a p-type silicon semiconductor, which may be single crystalline, polycrystalline or amorphous, is subjected to a diffusion, treatment on one surface with an n-type dopant to form a pn-junction and electrodes are provided, one in contact with the p-type region and the other in contact with the n-type diffusion layer. When the cell is irradiated on the surface of the retype diffusion layer with light such as sunlight, photovoltaic power is generated between the electrode in contact with the p-type region as the positive electrode and the electrode in contact with the n-type diffusion layer as the negative electrode.
A problem in the silicon solar cell of the above described structure is the high manufacturing cost of the silicon semiconductor substrate, especially due to the material loss in the process of slicing of a single crystalline or polycrystalline rod of silicon and lapping of the sliced wafers. In a silicon solar cell using amorphous silicon semiconductor in the form of a thin film, the cost for the material per se is very small but such silicon solar cells are still not practical for energy conversion because of the high investment costs for the manufacturing facilities and also of the degradation in the conversion efficiency of the cell with time as a result of an increase in the density of defects in the solar cells.